Flexible support structure for a geared architecture gas turbine engine

ABSTRACT

A gas turbine engine includes a fan shaft and a support which supports the fan shaft. The support defines a support lateral stiffness. A gear system drives the fan shaft. A flexible support at least partially supports the gear system, and defines a flexible support lateral stiffness with respect to the support lateral stiffness. An input to the gear system defines an input lateral stiffness with respect to the support lateral stiffness. A method of designing a gas turbine engine is also disclosed.

RELATED APPLICATION

The present disclosure is a continuation-in-part of U.S. patentapplication Ser. No. 13/623,309, filed Sep. 20, 2012, which is acontinuation-in-part of U.S. application Ser. No. 13/342,508, filed Jan.3, 2012, now U.S. Pat. No. 8,297,916, issued Oct. 30, 2012, whichclaimed priority to U.S. Provisional Application No. 61/494,453, filedJun. 8, 2011.

BACKGROUND

The present disclosure relates to a gas turbine engine, and moreparticularly to a flexible support structure for a geared architecturetherefor.

Epicyclic gearboxes with planetary or star gear trains may be used ingas turbine engines for their compact designs and efficient high gearreduction capabilities. Planetary and star gear trains generally includethree gear train elements: a central sun gear, an outer ring gear withinternal gear teeth, and a plurality of planet gears supported by aplanet carrier between and in meshed engagement with both the sun gearand the ring gear. The gear train elements share a common longitudinalcentral axis, about which at least two rotate. An advantage of epicyclicgear trains is that a rotary input can be connected to any one of thethree elements. One of the other two elements is then held stationarywith respect to the other two to permit the third to serve as an output.

In gas turbine engine applications, where a speed reduction transmissionis required, the central sun gear generally receives rotary input fromthe power plant, the outer ring gear is generally held stationary andthe planet gear carrier rotates in the same direction as the sun gear toprovide torque output at a reduced rotational speed. In star geartrains, the planet carrier is held stationary and the output shaft isdriven by the ring gear in a direction opposite that of the sun gear.

During flight, light weight structural cases deflect with aero andmaneuver loads causing significant amounts of transverse deflectioncommonly known as backbone bending of the engine. This deflection maycause the individual sun or planet gear's axis of rotation to loseparallelism with the central axis. This deflection may result in somemisalignment at gear train journal bearings and at the gear teeth mesh,which may lead to efficiency losses from the misalignment and potentialreduced life from increases in the concentrated stresses.

SUMMARY

In a featured embodiment, a gas turbine engine includes a fan shaft anda support which supports the fan shaft. The support defines a supportlateral stiffness. A gear system drives the fan shaft. A flexiblesupport at least partially supports the gear system, and defines aflexible support lateral stiffness with respect to the support lateralstiffness. An input to the gear system defines an input lateralstiffness with respect to the support lateral stiffness.

In another embodiment according to the previous embodiment, the supportand the flexible support are mounted to a static structure.

In another embodiment according to any of the previous embodiments, thesupport and the flexible support are mounted to a static structure of agas turbine engine.

In another embodiment according to any of the previous embodiments, thesupport and the flexible support are mounted to a front center body of agas turbine engine.

In another embodiment according to any of the previous embodiments, theflexible support is mounted to a planet carrier of the gear system.

In another embodiment according to any of the previous embodiments, theinput is mounted to a sun gear of the gear system.

In another embodiment according to any of the previous embodiments, thefan shaft is mounted to a ring gear of the gear system.

In another embodiment according to any of the previous embodiments, thegear system is a star system.

In another embodiment according to any of the previous embodiments, theflexible support is mounted to a ring gear of the gear system.

In another embodiment according to any of the previous embodiments, theinput is mounted to a sun gear of the gear system.

In another embodiment according to any of the previous embodiments, thefan shaft is mounted to a planet carrier of the gear system.

In another embodiment according to any of the previous embodiments, thegear system is a planet system.

In another embodiment according to any of the previous embodiments, alow speed spool drives the input.

In another embodiment according to any of the previous embodiments, theflexible support lateral stiffness and the input lateral stiffness areboth less than the support lateral stiffness.

In another embodiment according to any of the previous embodiments, atleast one of the flexible support lateral stiffness and the inputlateral stiffness is less than about 20% of the support lateralstiffness.

In another embodiment according to any of the previous embodiments, theflexible support lateral stiffness and the input lateral stiffness areeach less than about 20% of the support lateral stiffness.

In another embodiment according to any of the previous embodiments, atleast one of the flexible support lateral stiffness and the inputlateral stiffness is less than about 11% of the support lateralstiffness.

In another embodiment according to any of the previous embodiments, theflexible support lateral stiffness and the input lateral stiffness areeach less than about 11% of the support lateral stiffness.

In another embodiment according to any of the previous embodiments, aturbine provides an input to the gear system.

In another embodiment according to any of the previous embodiments, thegear system further drives a compressor rotor at a common speed with thefan shaft.

In another embodiment according to any of the previous embodiments, aturbine section drives the gear system and at least two compressorrotors. A fan drive turbine drives the gear system. At least two otherturbines drive at least two compressor rotors.

In another featured embodiment, a gas turbine engine includes a fanshaft and a support which supports the fan shaft. A gear system drivesthe fan shaft and includes a gear mesh that defines a gear mesh lateralstiffness. A flexible support at least partially supports the gearsystem, and defines a flexible support lateral stiffness with respect tothe gear mesh lateral stiffness. An input to the gear system defines aninput lateral stiffness with respect to the gear mesh lateral stiffness.

In another embodiment according to the previous embodiment, both theflexible support lateral stiffness and the input lateral stiffness areless than the gear mesh lateral stiffness.

In another embodiment according to any of the previous embodiments, theflexible support lateral stiffness is less than about 8% of the gearmesh lateral stiffness.

In another embodiment according to any of the previous embodiments, theinput lateral stiffness is less than about 5% of the gear mesh lateralstiffness.

In another embodiment according to any of the previous embodiments, alateral stiffness of a ring gear of the gear system is less than about20% of the gear mesh lateral stiffness.

In another embodiment according to any of the previous embodiments, alateral stiffness of a ring gear of the gear system is less than about12% of the gear mesh lateral stiffness.

In another embodiment according to any of the previous embodiments, alateral stiffness of a planet journal bearing which supports a planetgear of the gear system is less than or equal to the gear mesh lateralstiffness.

In another featured embodiment, a method of designing a gas turbineengine includes providing a fan shaft, and a support which supports thefan shaft. The support defines a support lateral stiffness. A gearsystem drives the fan shaft. A flexible support at least partiallysupports the gear system, and defines a flexible support lateralstiffness with respect to the support lateral stiffness. An inputdefines an input lateral stiffness with respect to the support lateralstiffness.

In another embodiment according to the previous embodiment, theembodiment includes a turbine section to drive the gear system and atleast two compressor rotors. A fan drive turbine drives the gear system.At least two other turbines drive at least two compressor rotors.

These and other features may be best understood from the followingdrawings and specification.

BRIEF DESCRIPTION OF THE DRAWINGS

Various features will become apparent to those skilled in the art fromthe following detailed description of the disclosed non-limitingembodiment. The drawings that accompany the detailed description can bebriefly described as follows:

FIG. 1 is a schematic cross-section of a gas turbine engine;

FIG. 2 is an enlarged cross-section of a section of the gas turbineengine which illustrates a fan drive gear system (FDGS);

FIG. 3 is a schematic view of a flex mount arrangement for onenon-limiting embodiment of the FDGS;

FIG. 4 is a schematic view of a flex mount arrangement for anothernon-limiting embodiment of the FDGS;

FIG. 5 is a schematic view of a flex mount arrangement for anothernon-limiting embodiment of a star system FDGS; and

FIG. 6 is a schematic view of a flex mount arrangement for anothernon-limiting embodiment of a planetary system FDGS.

FIG. 7 shows another embodiment.

FIG. 8 shows yet another embodiment.

DETAILED DESCRIPTION

FIG. 1 schematically illustrates a gas turbine engine 20. The gasturbine engine 20 is disclosed herein as a two-spool turbofan thatgenerally incorporates a fan section 22, a compressor section 24, acombustor section 26 and a turbine section 28. Alternative engines mightinclude an augmentor section (not shown) among other systems orfeatures. The fan section 22 drives air along a bypass flow path B in abypass duct defined within a nacelle 15, while the compressor section 24drives air along a core flow path C for compression and communicationinto the combustor section 26 then expansion through the turbine section28. Although depicted as a two-spool turbofan gas turbine engine in thedisclosed non-limiting embodiment, it should be understood that theconcepts described herein are not limited to use with two-spoolturbofans as the teachings may be applied to other types of turbineengines including three-spool architectures.

The exemplary engine 20 generally includes a low speed spool 30 and ahigh speed spool 32 mounted for rotation about an engine centrallongitudinal axis A relative to an engine static structure 36 viaseveral bearing systems 38. It should be understood that various bearingsystems 38 at various locations may alternatively or additionally beprovided, and the location of bearing systems 38 may be varied asappropriate to the application.

The low speed spool 30 generally includes an inner shaft 40 thatinterconnects a fan 42, a first (or low) pressure compressor 44 and afirst (or low) pressure turbine 46. The inner shaft 40 is connected tothe fan 42 through a speed change mechanism, which in exemplary gasturbine engine 20 is illustrated as a geared architecture 48 to drivethe fan 42 at a lower speed than the low speed spool 30. The high speedspool 32 includes an outer shaft 50 that interconnects a second (orhigh) pressure compressor 52 and a second (or high) pressure turbine 54.A combustor 56 is arranged in exemplary gas turbine 20 between the highpressure compressor 52 and the high pressure turbine 54. A mid-turbineframe 57 of the engine static structure 36 is arranged generally betweenthe high pressure turbine 54 and the low pressure turbine 46. Themid-turbine frame 57 further supports bearing systems 38 in the turbinesection 28. The inner shaft 40 and the outer shaft 50 are concentric androtate via bearing systems 38 about the engine central longitudinal axisA which is collinear with their longitudinal axes.

The core airflow is compressed by the low pressure compressor 44 thenthe high pressure compressor 52, mixed and burned with fuel in thecombustor 56, then expanded over the high pressure turbine 54 and lowpressure turbine 46. The mid-turbine frame 57 includes airfoils 59 whichare in the core airflow path C. The turbines 46, 54 rotationally drivethe respective low speed spool 30 and high speed spool 32 in response tothe expansion. It will be appreciated that each of the positions of thefan section 22, compressor section 24, combustor section 26, turbinesection 28, and fan drive gear system 48 may be varied. For example,gear system 48 may be located aft of combustor section 26 or even aft ofturbine section 28, and fan section 22 may be positioned forward or aftof the location of gear system 48.

The engine 20 in one example is a high-bypass geared aircraft engine. Ina further example, the engine 20 bypass ratio is greater than about six(6), with an example embodiment being greater than about ten (10), thegeared architecture 48 is an epicyclic gear train, such as a planetarygear system or other gear system, with a gear reduction ratio of greaterthan about 2.3 and the low pressure turbine 46 has a pressure ratio thatis greater than about five. In one disclosed embodiment, the engine 20bypass ratio is greater than about ten (10:1), the fan diameter issignificantly larger than that of the low pressure compressor 44, andthe low pressure turbine 46 has a pressure ratio that is greater thanabout five 5:1. Low pressure turbine 46 pressure ratio is pressuremeasured prior to inlet of low pressure turbine 46 as related to thepressure at the outlet of the low pressure turbine 46 prior to anexhaust nozzle. The geared architecture 48 may be an epicycle geartrain, such as a planetary gear system or other gear system, with a gearreduction ratio of greater than about 2.3:1. It should be understood,however, that the above parameters are only exemplary of one embodimentof a geared architecture engine and that the present invention isapplicable to other gas turbine engines including direct driveturbofans.

A significant amount of thrust is provided by the bypass flow B due tothe high bypass ratio. The fan section 22 of the engine 20 is designedfor a particular flight condition—typically cruise at about 0.8 Mach andabout 35,000 feet. The flight condition of 0.8 Mach and 35,000 ft, withthe engine at its best fuel consumption—also known as “bucket cruiseThrust Specific Fuel Consumption (‘TSFC’)”—is the industry standardparameter of 1 bm of fuel being burned divided by 1 bf of thrust theengine produces at that minimum point. “Low fan pressure ratio” is thepressure ratio across the fan blade alone, without a Fan Exit Guide Vane(“FEGV”) system. The low fan pressure ratio as disclosed hereinaccording to one non-limiting embodiment is less than about 1.45. “Lowcorrected fan tip speed” is the actual fan tip speed in ft/sec dividedby an industry standard temperature correction of [(Tram ° R)/(518.7°R)]^(0.5). The “Low corrected fan tip speed” as disclosed hereinaccording to one non-limiting embodiment is less than about 1150ft/second.

With reference to FIG. 2, the geared architecture 48 generally includesa fan drive gear system (FDGS) 60 driven by the low speed spool 30(illustrated schematically) through an input 62. The input 62, which maybe in the form of a coupling, both transfers torque from the low speedspool 30 to the geared architecture 48 and facilitates the segregationof vibrations and other transients therebetween. In the disclosednon-limiting embodiment, the FDGS 60 may include an epicyclic gearsystem which may be, for example, a star system or a planet system.

The input coupling 62 may include an interface spline 64 joined, by agear spline 66, to a sun gear 68 of the FDGS 60. The sun gear 68 is inmeshed engagement with multiple planet gears 70, of which theillustrated planet gear 70 is representative. Each planet gear 70 isrotatably mounted in a planet carrier 72 by a respective planet journalbearing 75. Rotary motion of the sun gear 68 urges each planet gear 70to rotate about a respective longitudinal axis P.

Each planet gear 70 is also in meshed engagement with rotating ring gear74 that is mechanically connected to a fan shaft 76. Since the planetgears 70 mesh with both the rotating ring gear 74 as well as therotating sun gear 68, the planet gears 70 rotate about their own axes todrive the ring gear 74 to rotate about engine axis A. The rotation ofthe ring gear 74 is conveyed to the fan 42 (FIG. 1) through the fanshaft 76 to thereby drive the fan 42 at a lower speed than the low speedspool 30. It should be understood that the described geared architecture48 is but a single non-limiting embodiment and that various other gearedarchitectures will alternatively benefit herefrom.

With reference to FIG. 3, a flexible support 78 supports the planetcarrier 72 to at least partially support the FDGS 60A with respect tothe static structure 36 such as a front center body which facilitatesthe segregation of vibrations and other transients therebetween. Itshould be understood that various gas turbine engine case structures mayalternatively or additionally provide the static structure and flexiblesupport 78. It should be understood that lateral as defined herein isgenerally transverse to the axis of rotation A which typically absorbstransverse deflection which may be otherwise applied to the FDGS 60. Thestatic structure 36 may further include a number 1 and 1.5 bearingsupport static structure 82 which is commonly referred to as a “K-frame”which supports the number 1 and number 1.5 bearing systems 38A. 38B.Notably, the K-frame bearing support defines a lateral stiffness(Kframe) as the referenced factor in this non-limiting embodiment.

In this disclosed non-limiting embodiment, the lateral stiffness (KFS;KIC) of both the flexible support 78 and the input coupling 62 are eachless than about 11% of the lateral stiffness (Kframe). That is, thelateral stiffness of the entire FDGS 60 is controlled by thisrelationship.

With reference to FIG. 4, another non-limiting embodiment of a FDGS 60Bincludes a flexible support 78′ that supports a rotationally fixed ringgear 74′. The fan shaft 76′ is driven by the planet carrier 72′ in theschematically illustrated planet system which otherwise generallyfollows the star system architecture of FIG. 3.

With reference to FIG. 5, the lateral stiffness relationship within aFDGS 60C itself (for a star system architecture) is schematicallyrepresented. The lateral stiffness (KIC) of an input coupling 62, alateral stiffness (KFS) of a flexible support 78, a lateral stiffness(KRG) of a ring gear 74 and a lateral stiffness (KJB) of a planetjournal bearing 75 are controlled with respect to a lateral stiffness(KGM) of a gear mesh within the FDGS 60.

In the disclosed non-limiting embodiment, the stiffness (KGM) may bedefined by the gear mesh between the sun gear 68 and the multiple planetgears 70. The lateral stiffness (KGM) within the FDGS 60 is thereferenced factor and the static structure 82′ rigidly supports the fanshaft 76. That is, the fan shaft 76 is supported upon bearing systems38A, 38B which are essentially rigidly supported by the static structure82′. The lateral stiffness (KJB) may be mechanically defined by, forexample, the stiffness within the planet journal bearing 75 and thelateral stiffness (KRG) of the ring gear 74 may be mechanically definedby, for example, the geometry of the ring gear wings 74L, 74R (FIG. 2).

In the disclosed non-limiting embodiment, the lateral stiffness (KRG) ofthe ring gear 74 is less than about 12% of the lateral stiffness (KGM)of the gear mesh; the lateral stiffness (KFS) of the flexible support 78is less than about 8% of the lateral stiffness (KGM) of the gear mesh;the lateral stiffness (KJB) of the planet journal bearing 75 is lessthan or equal to the lateral stiffness (KGM) of the gear mesh; and thelateral stiffness (KIC) of an input coupling 62 is less than about 5% ofthe lateral stiffness (KGM) of the gear mesh.

With reference to FIG. 6, another non-limiting embodiment of a lateralstiffness relationship within a FDGS 60D itself are schematicallyillustrated for a planetary gear system architecture, which otherwisegenerally follows the star system architecture of FIG. 5.

It should be understood that combinations of the above lateral stiffnessrelationships may be utilized as well. The lateral stiffness of each ofstructural components may be readily measured as compared to filmstiffness and spline stiffness which may be relatively difficult todetermine.

By flex mounting to accommodate misalignment of the shafts under designloads, the FDGS design loads have been reduced by more than 17% whichreduces overall engine weight. The flex mount facilitates alignment toincrease system life and reliability. The lateral flexibility in theflexible support and input coupling allows the FDGS to essentially‘float’ with the fan shaft during maneuvers. This allows: (a) the torquetransmissions in the fan shaft, the input coupling and the flexiblesupport to remain constant during maneuvers; (b) maneuver inducedlateral loads in the fan shaft (which may otherwise potentially misaligngears and damage teeth) to be mainly reacted to through the number 1 and1.5 bearing support K-frame; and (c) both the flexible support and theinput coupling to transmit small amounts of lateral loads into the FDGS.The splines, gear tooth stiffness, journal bearings, and ring gearligaments are specifically designed to minimize gear tooth stressvariations during maneuvers. The other connections to the FDGS areflexible mounts (turbine coupling, case flex mount). These mount springrates have been determined from analysis and proven in rig and flighttesting to isolate the gears from engine maneuver loads. In addition,the planet journal bearing spring rate may also be controlled to supportsystem flexibility.

FIG. 7 shows an embodiment 200, wherein there is a fan drive turbine 208driving a shaft 206 to in turn drive a fan rotor 202. A gear reduction204 may be positioned between the fan drive turbine 208 and the fanrotor 202. This gear reduction 204 may be structured, mounted andoperate like the gear reduction disclosed above. A compressor rotor 210is driven by an intermediate pressure turbine 212, and a second stagecompressor rotor 214 is driven by a turbine rotor 216. A combustionsection 218 is positioned intermediate the compressor rotor 214 and theturbine section 216.

FIG. 8 shows yet another embodiment 300 wherein a fan rotor 302 and afirst stage compressor 304 rotate at a common speed. The gear reduction306 (which may be structured, mounted and operate as disclosed above) isintermediate the compressor rotor 304 and a shaft 308, which is drivenby a low pressure turbine section.

It should be understood that relative positional terms such as“forward,” “aft,” “upper,” “lower,” “above,” “below,” and the like arewith reference to the normal operational attitude of the vehicle andshould not be considered otherwise limiting.

It should be understood that like reference numerals identifycorresponding or similar elements throughout the several drawings. Itshould also be understood that although a particular componentarrangement is disclosed in the illustrated embodiment, otherarrangements will benefit herefrom.

Although particular step sequences are shown, described, and claimed, itshould be understood that steps may be performed in any order, separatedor combined unless otherwise indicated and will still benefit from thepresent disclosure.

The foregoing description is exemplary rather than defined by thelimitations within. Various non-limiting embodiments are disclosedherein, however, one of ordinary skill in the art would recognize thatvarious modifications and variations in light of the above teachingswill fall within the scope of the appended claims. It is therefore to beunderstood that within the scope of the appended claims, the disclosuremay be practiced other than as specifically described. For that reasonthe appended claims should be studied to determine true scope andcontent.

Although an embodiment of this invention has been disclosed, a worker ofordinary skill in this art would recognize that certain modificationswould come within the scope of this disclosure. For that reason, thefollowing claims should be studied to determine the true scope andcontent of this disclosure.

The invention claimed is:
 1. A gas turbine engine comprising: a fanshaft; a support which supports the fan shaft, the support defines asupport lateral stiffness; a gear system to drive the fan shaft; aflexible support which at least partially supports the gear system, theflexible support defines a flexible support lateral stiffness withrespect to the support lateral stiffness; and an input to the gearsystem, the input defining an input lateral stiffness with respect tothe support lateral stiffness.
 2. The gas turbine engine as recited inclaim 1, wherein the support and the flexible support are mounted to astatic structure.
 3. The gas turbine engine as recited in claim 1,wherein the support and the flexible support are mounted to a frontcenter body of a gas turbine engine.
 4. The gas turbine engine asrecited in claim 1, wherein the flexible support is mounted to a planetcarrier of the gear system.
 5. The gas turbine engine as recited inclaim 4, wherein the input is mounted to a sun gear of the gear system.6. The gas turbine engine as recited in claim 5, wherein the fan shaftis mounted to a ring gear of the gear system.
 7. The gas turbine engineas recited in claim 6, wherein the gear system is a star system.
 8. Thegas turbine engine as recited in claim 1, wherein the flexible supportis mounted to a ring gear of the gear system.
 9. The gas turbine engineas recited in claim 8, wherein the input is mounted to a sun gear of thegear system.
 10. The gas turbine engine as recited in claim 9, whereinthe fan shaft is mounted to a planet carrier of the gear system.
 11. Thegas turbine engine as recited in claim 10, wherein the gear system is aplanet system.
 12. The gas turbine engine as recited in claim 1, furthercomprising a low speed spool for driving the input.
 13. The gas turbineengine as recited in claim 1, wherein the flexible support lateralstiffness and the input lateral stiffness are both less than the supportlateral stiffness.
 14. The gas turbine engine as recited in claim 13,wherein at least one of the flexible support lateral stiffness and theinput lateral stiffness is less than about 20% of the support lateralstiffness.
 15. The gas turbine engine as recited in claim 14, whereinthe flexible support lateral stiffness and the input lateral stiffnessare each less than about 20% of the support lateral stiffness.
 16. Thegas turbine engine as recited in claim 13, wherein at least one of theflexible support lateral stiffness and the input lateral stiffness isless than about 11% of the support lateral stiffness.
 17. The gasturbine engine as recited in claim 16, wherein the flexible supportlateral stiffness and the input lateral stiffness are each less thanabout 11% of the support lateral stiffness.
 18. The gas turbine engineas recited in claim 1, wherein a turbine provides an input to the gearsystem, and the gear system further drives a compressor rotor at acommon speed with the fan shaft.
 19. The gas turbine engine as recitedin claim 1, comprising a turbine section to drive the gear system and atleast two compressor rotors, wherein the turbine section includes a fandrive turbine to drive the gear system and at least two other turbinesto drive the at least two compressor rotors.
 20. A gas turbine enginecomprising: a fan shaft; a support which supports the fan shaft; a gearsystem to drive the fan shaft, the gear system includes a gear mesh thatdefines a gear mesh lateral stiffness; a flexible support which at leastpartially supports the gear system, the flexible support defining aflexible support lateral stiffness with respect to the gear mesh lateralstiffness; and an input to the gear system, the input defining an inputlateral stiffness with respect to the gear mesh lateral stiffness. 21.The gas turbine engine as recited in claim 20, wherein both the flexiblesupport lateral stiffness and the input lateral stiffness are less thanthe gear mesh lateral stiffness.
 22. The gas turbine engine as recitedin claim 21, wherein the flexible support lateral stiffness is less thanabout 8% of the gear mesh lateral stiffness.
 23. The gas turbine engineas recited in claim 21, wherein the input lateral stiffness is less thanabout 5% of the gear mesh lateral stiffness.
 24. The gas turbine engineas recited in claim 20, wherein a lateral stiffness of a ring gear ofthe gear system is less than about 20% of the gear mesh lateralstiffness.
 25. The gas turbine engine as recited in claim 20, wherein alateral stiffness of a ring gear of the gear system is less than about12% of the gear mesh lateral stiffness.
 26. The gas turbine engine asrecited in claim 20, wherein a lateral stiffness of a planet journalbearing which supports a planet gear of the gear system is less than orequal to the gear mesh lateral stiffness.
 27. A method of designing agas turbine engine comprising: providing a fan shaft; providing asupport which supports the fan shaft, the support defining a supportlateral stiffness; providing a gear system to drive the fan shaft;providing a flexible support which at least partially supports the gearsystem, the flexible support defining a flexible support lateralstiffness with respect to the support lateral stiffness; and providingan input to the gear system, the input defining an input lateralstiffness with respect to the support lateral stiffness.
 28. The methodas recited in claim 27, comprising a turbine section to drive the gearsystem and at least two compressor rotors, wherein the turbine sectionincludes a fan drive turbine to drive the gear system and at least twoother turbines to drive the at least two compressor rotors.
 29. A methodof designing a gear system for a gas turbine engine comprising:designing an output shaft; designing a support which supports the outputshaft, the support defining at least one of a support transversestiffness and a support lateral stiffness; designing a gear assembly todrive the output shaft; designing a flexible support which at leastpartially supports the gear assembly, the flexible support defining atleast one of a flexible support transverse stiffness and a flexiblesupport lateral stiffness with respect to the at least one of thesupport transverse stiffness and the support lateral stiffness; anddesigning an input to the gear assembly, the input defining at least oneof an input transverse stiffness and an input lateral stiffness withrespect to the at least one of the support transverse stiffness and thesupport lateral stiffness.